Electrochemical process for preparation of zinc metal

ABSTRACT

Disclosed is a low corrosion electrochemical process for preparing zinc metal which comprises electrochemically reducing an aqueous basic solution or slurry of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, wherein the electrochemical process is carried out in an undivided electrochemical cell, and wherein air or nitrogen is bubbled in through the solution or slurry of zinc oxide or said other zinc compound during said electrochemical process.

FIELD OF THE INVENTION

[0001] The present invention provides an electrochemical process for the preparation of zinc metal.

BACKGROUND OF THE INVENTION

[0002] Zinc powder is widely used in the chemical industry in various industries. Zinc oxide containing other zinc salts, metal impurities, etc. is produced as a byproduct. Recycling of the zinc oxide to produce pure zinc powder is highly desirable from a cost as well as an environmental point of view.

[0003] The electrodeposition of zinc metal is a well-known reaction in electrochemical technology (See, for example, D. Pletcher and F. C. Walsh, Industrial Electrochemistry, Blackie Academic, 1993). The electrogalvanizing of steel is a process carried out on a very large scale and aqueous acid is the normal medium. High speed, reel to reel galvanizing of steel is carried out in sulfuric acid with dimensionally stable anodes and uniform deposition is achieved at high current density by inducing very efficient mass transport by rapid movement of the steel surface. The deposition of zinc metal is also the critical electrode reaction in the electrowinning and electrorefining of zinc. In addition, there are a number of technologies, which have been demonstrated for the removal of Zn(II) from effluents. However, in these technologies, concentration of Zn(II) is low, commonly less than 100 ppm. Finally, the deposition of zinc has been widely investigated as the cathodic reaction in candidate secondary batteries. In all these applications, however, the objective is to select the conditions so as to give an adhesive and smooth zinc coating.

[0004] Zinc metal can be produced by electrolysis either in strong alkaline or neutral zinc containing solutions. The first patents obtained on the alkaline electrolysis process date back to the early thirties (German Patents, 581013, 506590, 653557). In these methods, a low current density of 1200-1500 amperes/sq. meter (A/m²) was used. Volume efficiency and current density of these batch type processes are too low to be industrially attractive. I. Orszagh and B. Vass (Hung. J. Ind. Chem., 13, (1985) 287) used these methods to recycle zinc oxide byproduct from zinc dithionite production.

[0005] There is at present a need to carry out an electrolysis reaction to produce zinc metal under conditions where corrosion of the electrodes is minimized. The present invention fulfils this and other needs. The present invention provides a low corrosion electrochemical process for preparing zinc metal wherein air or nitrogen is bubbled in during the electrochemical reduction process producing the zinc metal. It has been unexpectedly found that bubbling of air or nitrogen reduces electrode corrosion during the electrochemical process.

[0006] U.S. Pat. No. 5,958,210 discloses a method for electrowinning metallic zinc from zinc ion in aqueous solution, said method comprising performing electrolysis on a mixture of solid conductive particles and aqueous alkali solution, said solution ranging in concentration from about 3N to about 20N alkali and containing dissolved zinc ion at an initial concentration ranging from about 50 to about 500 grams of zinc ion per liter of said solution, in an electrolytic cell containing first and second vertically arranged, parallel flat plates defined as a current feeder and a counter electrode, respectively, said counter electrode coated with a substance that is catalytic for oxygen evolution, said cell further containing an ion-permeable diaphragm parallel to each of said plates and interposed therebetween to define a gap between said current feeder and said diaphragm, by passing said mixture of particles and solution through said gap such that said particles contact said current feeder and passing a current across said gap, thereby depositing metallic zinc from said solution onto said particles.

[0007] U.S. patent application Ser. No. 09/776,518 (filed Feb. 2, 2001) discloses an electrochemical process for preparing zinc powder which involves: a) providing to an electrochemical cell a basic solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the basic solution prepared by dissolving the zinc oxide or the other zinc compound in an aqueous 2.5 to 10.0 M base solution; and b) passing current to the cell at a current density of about 10,000 to about 40,000 A/m² for a time period sufficient to electrochemically reduce the zinc oxide to zinc powder, wherein the electrochemical process has a current efficiency of at least 70% and is substantially free from electrode corrosion.

[0008] U.S. patent application Ser. No. 09/776,644 (filed Feb. 2, 2001) discloses a continuous electrochemical process for preparing zinc powder which involves: providing to an electrochemical cell a solution or suspension in an aqueous 1.25 Molar to 10.0 Molar base solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the solution or suspension containing at least 2 millimoles of solubilized zinc based species per 100 grams of electrolyte; and b) passing current to the cell at a current density of about 500 to 40,000 A/m², for a time period sufficient to electrochemically reduce the solubilized zinc based species to zinc powder, while continuously or intermittently adding a sufficient amount of the zinc oxide or the other zinc compound to the cell to maintain the concentration of the solubilized zinc based species at a level of at least 2 millimoles per 100 grams of electrolyte and continuously or intermittently removing at least a portion of the zinc powder formed; wherein the electrolyte includes the aqueous base solution and the zinc oxide or the other zinc compound.

SUMMARY OF THE INVENTION

[0009] The present invention provides a low corrosion electrochemical process for preparing zinc metal which comprises electrochemically reducing an aqueous basic solution or slurry of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, wherein the electrochemical process is carried out in an undivided electrochemical cell, and wherein air or nitrogen is bubbled in through the solution or slurry of zinc oxide or said other zinc compound during said electrochemical process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The present invention provides a low corrosion electrochemical process for preparing zinc metal which comprises electrochemically reducing an aqueous basic solution or slurry of zinc oxide or any other zinc compound (such as zinc sulfate) that reacts with an aqueous base to produce zinc oxide, wherein the electrochemical process is carried out in an undivided electrochemical cell, and wherein air or nitrogen is bubbled in through the solution or slurry of zinc oxide or said other zinc compound during said electrochemical process.

[0011] The anode may be made from any conventional suitable material such as platinum, or iridium, either of which may be coated over an inert support such as niobium or titanium. The anode may also be made of nickel, or from conventional materials having good alkali corrosion resistance, e.g., lead or stainless steel. The cathode may be made from any conventional suitable materials having good alkali corrosion resistance, such as magnesium, magnesium alloy, copper, lead and stainless steel. Preferably, the anode in the present invention is formed of stainless steel or nickel and the cathode is formed of stainless steel, magnesium, magnesium alloy or copper. In one embodiment, the cathode and the anode are stainless steel and copper respectively, and in one embodiment nickel and copper respectively.

[0012] The aqueous base solutions employed in the process of the invention are prepared by combining water with a source of alkali metal or alkaline earth metal ions, such as lithium, sodium, and potassium, and a source of hydroxyl (OH⁻ ions). A single source may of course provide both types of ions. The various alkali or alkaline earth metal ions are preferably supplied from various compounds such as hydroxides and oxides. Preferred base solutions are sodium and potassium hydroxide solutions.

[0013] For electrolysis, temperatures higher than ambient are generally desired because of the beneficial effects on the kinetics of all steps in an electrode process. At higher temperatures, the diffusion coefficient, the exchange current density and the rates of chemical reactions generally are increased. The decrease in viscosity and increase in diffusion coefficient leads to the increased mass transport rates. This increased mass transport of zinc ions from the bulk of the solution to the cathodic region is highly desirable. However, increase in the rate of chemical reaction such as the oxidation of zinc produced with oxygen and mass transport of the byproduct oxygen to the bulk of the solution may not be desirable. In the present invention, higher than ambient temperatures are found to be favorable for the electrolytic reduction of zinc oxide to zinc, and are preferred.

[0014] In one embodiment, the presently claimed electrochemical reduction process is conducted at a temperature of from 110° C. to 105° C., preferably from 40° to 80° C., and more preferably from 60° to 75° C.

[0015] In one embodiment, the electrochemical process of the present invention is a continuous process. The meaning of “continuous” in the present context is well understood by one of ordinary skill in the art. As used herein, it relates a process wherein zinc oxide or the other zinc compound can be added continuously to the electrochemical cell and at least a portion of the zinc metal formed is removed continuously or intermittently during the electrochemical process. A continuous electrochemical process has been disclosed in U.S. application Ser. No. 09/776,664 (filed Feb. 2, 2001).

[0016] The following specific examples will provide detailed illustrations of the methods of producing and utilizing compositions of the present invention. These examples are not intended, however, to limit or restrict the scope of the invention in any way and should not be construed as providing conditions, parameters or values which must be utilized exclusively in order to practice the present invention. Unless otherwise specified, all parts and percents are by weight.

EXAMPLES Example 1

[0017] General Procedure Used for Electrolytic Reduction of Zinc Oxide to Zinc Metal:

[0018] In these experiments, a resin Kettle (5 inch in diameter and 18 inch high) is used as the cell. A solution or slurry of zinc oxide in the aqueous sodium hydroxide solution (3 to 3.5 liters) at 20 to 80° C. is charged into the resin kettle. A thermometer, desired cathodes and anodes are positioned in the cell using laboratory clamps. Mixing is achieved by bubbling air or nitrogen through the solution or slurry of ZnO. In some experiments mechanical stirring in addition to bubbling of air or nitrogen is used. Control experiments use only mechanical stirring for mixing (no air or nitrogen bubbling). Parts of the cathode and anode surfaces are covered with Teflon tape to achieve the desired active cathode and anode surface areas. Electrolysis is carried out at a current density of about 5000 Amps/m². A portion of the zinc deposited on the cathode is removed periodically. At the end of the experiment, zinc particles are separated from the electrolyte by decantation, washed with water and then dried. Dried zinc particles were analyzed to determine the zinc content.

Example 2

[0019] The results of electrolysis of zinc oxide under various conditions are shown below in Table 1. TABLE 1 Zinc Current Anode Cathode Current NaOH Soln/ Cathode/ Moles formed Efficiency Corrosion Corrosion Density (Wt%) Slurry Anode4 Mixing electrons (moles) (%) g/45.5 kg Zn g/45.5 kg Zn (Amps/m²) 1 25 Soln Mg/SS Mech 3.66 1.63 89 6.8 0.3 5000 2 25 Soln Mg/SS N₂ 5.39 2.43 90 1.0 0.3 5000 3 26 Soln Mg/SS N₂ 4.56 2.19 96 0.0 0.0 5050 4 26 Soln Mg/SS Air 4.56 2.23 98 3.2 0.0 5050 5 25 Soln Cu/SS N₂ 4.56 2.10 92 3.8 0.0 5000 6 24 Slurry Cu/SS N₂ 7.26 3.58 99 4.8 1.0 5000 7 25 Slurry Cu/SS N_(2/mech) 5.61 2.62 94 4.7 0.2 5053 8 29 Slurry Cu/Ni mech 7.26 3.59 99 5.8 2.7 5000 9 29 Slurry Cu/Ni N_(2/mech) 7.26 3.50 96 0.0 0.2 5000 10 26 Slurry Cu/Ni mech 7.26 3.49 96 1.4 0.1 5000 11 25 Slurry Cu/Ni N_(2/mech) 5.82 2.74 94 0.3 0.1 5053

[0020] The data in Table 1 clearly indicate that electrode corrosion is diminished when nitrogen or air is bubbled through the reactor during the electrochemical process (compared to the control process when only mechanical stirring is used). Furthermore, the use of mechanical stirring in conjunction with nitrogen or air bubbling does not adversely affect electrode corrosion

[0021] Each of the documents referred to above is incorporated herein by reference in its entirety, for all purposes. Except in the Examples, or where otherwise explicitly indicated, all numerical quantities in this description specifying amounts and concentrations of materials, reaction and process conditions (such as temperature), and the like are to be understood to be modified by the word “about”.

[0022] While the invention has been explained in relation to its preferred embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims. 

1. A low corrosion electrochemical process for preparing zinc metal which comprises electrochemically reducing an aqueous basic solution or slurry of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, wherein the electrochemical process is carried out in an undivided electrochemical cell, and wherein air or nitrogen is bubbled in through the solution or slurry of zinc oxide or said other zinc compound during said electrochemical process.
 2. The electrochemical process of claim 1 that is a continuous process.
 3. The electrochemical process of claim 1 that utilizes an electrochemical cell having a magnesium or copper cathode.
 4. The electrochemical process of claim 1 that utilizes an electrochemical cell having a stainless steel or nickel anode.
 5. The electrochemical process of claim 1 that utilizes an electrochemical cell having a stainless steel anode and a copper cathode.
 6. The electrochemical process of claim 1 that utilizes an electrochemical cell having a nickel anode and a copper cathode.
 7. The electrochemical process of claim 1 that is conducted at a temperature of from about 10° C. to about 105° C.
 8. The electrochemical process of claim 7, wherein the temperature ranges from about 40° to about 80° C.
 9. The electrochemical process of claim 7, wherein the temperature ranges from about 60° to about 75° C.
 10. The electrochemical process of claim 1, wherein the aqueous base comprises ions of at least one alkali or alkaline earth metal and hydroxyl (OH⁻) ions.
 11. The electrochemical process of claim 10, wherein the alkali and alkaline earth metal ions are selected from sodium, potassium, and mixtures thereof and are provided in the form of a compound selected from hydroxides, and oxides.
 12. The electrochemical process of claim 11, wherein the compound is selected from the group consisting of sodium hydroxide and potassium hydroxide. 